The FBI said Friday that investigators now believe friendly fire led to the killing of Border Patrol Agent Nicholas Ivie and the wounding of a second agent earlier this week in southern Arizona, a stunning development in a case that had renewed concerns over cross-border drug-cartel violence and reignited a debate over border security.

One year ago this week, Border Patrol Agent Brian Terry was killed in a shootout in a remote smuggling area near here. Since then, federal authorities have ramped up the number of agents patrolling the entire southwestern border, particularly the region where he was shot.